Page replacement algorithm
In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out, sometimes called swap out, or write to disk, when a page of memory needs to be allocated. Page replacement happens when a requested page is not in memory (page fault) and a free page cannot be used to satisfy the allocation, either because there are none, or because the number of free pages is lower than some threshold.
When the page that was selected for replacement and paged out is referenced again it has to be paged in (read in from disk), and this involves waiting for I/O completion. This determines the quality of the page replacement algorithm: the less time waiting for page-ins, the better the algorithm. A page replacement algorithm looks at the limited information about accesses to the pages provided by hardware, and tries to guess which pages should be replaced to minimize the total number of page misses, while balancing this with the costs (primary storage and processor time) of the algorithm itself.
Local vs. global replacement
Replacement algorithms can be local or global.
When a process incurs a page fault, a local page replacement algorithm selects for replacement some page that belongs to that same process (or a group of processes sharing a memory partition). A global replacement algorithm is free to select any page in memory.
Local page replacement assumes some form of memory partitioning that determines how many pages are to be assigned to a given process or a group of processes. Most popular forms of partitioning are fixed partitioning and balanced set algorithms based on the working set model. The advantage of local page replacement is its scalability: each process can handle its page faults independently, leading to more consistent performance for that process. However global page replacement is more efficient on an overall system basis.
Detecting which pages are referenced and modified
Modern general purpose computers and some embedded processors have support for virtual memory. Each process has its own virtual address space. A page table maps a subset of the process virtual addresses to physical addresses. In addition, in most architectures the page table holds an "access" bit and a "dirty" bit for each page in the page table. The CPU sets the access bit when the process reads or writes memory in that page. The CPU sets the dirty bit when the process writes memory in that page. The operating system can modify the access and dirty bits. The operating system can detect accesses to memory and files through the following means:
By clearing the access bit in pages present in the process' page table. After some time, the OS scans the page table looking for pages that had the access bit set by the CPU. This is fast because the access bit it set automatically by the CPU and inaccurate because the OS does not immediately receives notice of the access nor does it have information about the order in which the process accessed these pages.
By removing pages from the process' page table without necessarily removing them from physical memory. The next access to that page is detected immediately because it causes a page fault. This is slow because a page fault involves a context switch to the OS, software lookup for the corresponding physical address, modification of the page table and a context switch back to the process and accurate because the access is detected immediately after it occurs.
Directly when the process makes system calls that potentially access the page cache like read and write in POSIX.
Most replacement algorithms simply return the target page as their result. This means that if target page is dirty (that is, contains data that have to be written to the stable storage before page can be reclaimed), I/O has to be initiated to send that page to the stable storage (to clean the page). In the early days of virtual memory, time spent on cleaning was not of much concern, because virtual memory was first implemented on systems with full duplex channels to the stable storage, and cleaning was customarily overlapped with paging. Contemporary commodity hardware, on the other hand, does not support full duplex transfers, and cleaning of target pages becomes an issue.
To deal with this situation, various precleaning policies are implemented. Precleaning is the mechanism that starts I/O on dirty pages that are (likely) to be replaced soon. The idea is that by the time the precleaned page is actually selected for the replacement, the I/O will complete and the page will be clean. Precleaning assumes that it is possible to identify pages that will be replaced next. Precleaning that is too eager can waste I/O bandwidth by writing pages that manage to get re-dirtied before being selected for replacement.
Demand Paging Basic concept
Demand paging follows that pages should only be brought into memory if the executing process demands them. This is often referred to as lazy evaluation as only those pages demanded by the process are swapped from secondary storage to main memory. Contrast this to pure swapping, where all memory for a process is swapped from secondary storage to main memory during the process startup.
Commonly, to achieve this process a page table implementation is used. The page table maps logical memory to physical memory. The page table uses a bitwise operator to mark if a page is valid or invalid. A valid page is one that currently resides in main memory. An invalid page is one that currently resides in secondary memory. When a process tries to access a page, the following steps are generally followed:
- Attempt to access page.
- If page is valid (in memory) then continue processing instruction as normal.
- If page is invalid then a page-fault trap occurs.
- Check if the memory reference is a valid reference to a location on secondary memory. If not, the process is terminated (illegal memory access). Otherwise, we have to page in the required page.
- Schedule disk operation to read the desired page into main memory.
- Restart the instruction that was interrupted by the operating system trap.
Demand paging, as opposed to loading all pages immediately:
- Only loads pages that are demanded by the executing process.
- As there is more space in main memory, more processes can be loaded, reducing the context switching time, which utilizes large amounts of resources.
- Less loading latency occurs at program startup, as less information is accessed from secondary storage and less information is brought into main memory.
- As main memory is expensive compared to secondary memory, this technique helps significantly reduce the bill of material (BOM) cost in smart phones for example. Symbian OS had this feature.
- Individual programs face extra latency when they access a page for the first time.
- Low-cost, low-power embedded systems may not have a memory management unit that supports page replacement.
- Memory management with page replacement algorithms becomes slightly more complex.
- Possible security risks, including vulnerability to timing attacks; see Percival, Colin (2005-05-13). "Cache missing for fun and profit" (PDF). BSDCan 2005. (specifically the virtual memory attack in section 2).
- Thrashing which may occur due to repeated page faults.
Some systems attempt to reduce latency of Demand paging by guessing which pages not in RAM are likely to be needed soon, and pre-loading such pages into RAM, before that page is requested. (This is often in combination with pre-cleaning, which guesses which pages currently in RAM are not likely to be needed soon, and pre-writing them out to storage).
When a page fault occurs, "anticipatory paging" systems will not only bring in the referenced page, but also the next few consecutive pages (analogous to a prefetch input queue in a CPU).
The swap prefetch mechanism goes even further in loading pages (even if they are not consecutive) that are likely to be needed soon.
"Virtual memory" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0
"Demand paging" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0
"Memory paging" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0
"Page replacement algorithm" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0